we transitioned our son at 35 months (he'd been in the same place since he was 3 months old!), and it was tough at first. we use a center-type setting, so our experience may be a bit different from an in-home.
we took him to visit "new school" before he started there by himself, and he liked it. there were things we could talk to him about that he remembered and enjoyed, so that helped (a favorite place to play, a neat toy, the teachers, etc.).
he was a basketcase for about 2 weeks (he only goes 3 days a week), but i stayed positive, planned some extra time with him at drop-off and he got really close to the early morning teacher. the staff was amazing - kept close to him, gave him lots of hugs & reassurance and helped him make new friends. we also ended up letting him bring a comfort item (or two) everyday - usually his stuffed cat & a blanket. it helped him feel less alone when i had to leave for work.
his upset blew over really pretty quickly. he would cry as i was leaving, but it lasted very few minutes. by the time he came home at night, he was excited about what he'd done during the day & talked non-stop about his day. i knew it was a good fit, so it made the drop-off drama a little easier to stomach. our previous provider had maintained that "quick drops" were best because the child recovered from upset faster. that never worked for me or felt comfortable, so i would stay until he was a little more calm... usually only about 5-10 minutes, though, so it wasn't like i was hanging out for half an hour or something.
he was in a not-so-hot environment before (their preschool program was ridiculous), so we had to make a change. he still misses his friends from "old school" but he never asks to go back. we went back to old school to play last week & visit - he actually said, "i miss my friends, but that's all. i really like new school. thanks, mom." :) so, in our experience, it gets a lot better.
mom to chase (3)